Recently, importance of green energy has come up as a solution to overcome exhaustion of fossil fuel and problem of global warming, and studies regarding thereto have been actively processed. Specifically, studies on green energy are progressed in two discriminate fields: the one is energy development due to natural phenomenon such as solar energy, wind power and tidal power; and the other is development of novel recycled energy such as bio-energy and hydrogen.
The most significant requisites for energy nowadays include 1) environmental friendliness, 2) reversible conversion and recycling, 3) applicability to motor vehicles such as automobiles and aircraft, and generating facilities in a large scale, and 4) availability to any place any time.
Hydrogen energy, that meets best the four requisites among green energies, is investigated in three fields: production, storage and application of hydrogen. In the field of hydrogen storage, it is focused to develop media that 1) satisfies 6 wt % of capacity suggested by DOE of United States as guideline in 2010; and 2) safely and reversibly stores a large volume of hydrogen.
Though hydrogen storage media suggested by a plurality of investigation groups are very diverse and complicated, they can be classified, depending upon the adsorption mechanism of hydrogen on the hydrogen storage media and the relevant adsorption energy range, into three: physisorption, Kubas adsorption and chemisorption.
The hydrogen storage materials by physisorption include porous substances having micro/medium pore, such as microporous (2 nm or less) and mesoporous (2˜50 nm) substances, which are further classified into carbon substances, inorganic oxides and metal-organic frameworks (MOF). The metal-organic framework essentially comprises metal salt and organic linker as the backbone. The coordinate structure formed by metal ion and organic linker may be a simple molecule formed by self-assembly, or in various forms such as one-dimensional (linear), two-dimensional (planar) and complicated three-dimensional structures. Since the porous materials store hydrogen in the pores of the material, they adsorb hydrogen in a proportional relevancy to the surface area (adsorption type I), having weak adsorption energy of not more than about 10 KJ/mol. In order to increase such weak adsorption energy (10 KJ/mol) to enable adsorption of a significant amount of hydrogen even at extremely low temperature or higher, doping with various precious metal (Pt, Pd, Ru, Rh) or transition metal (Co, Ni, Ti) has been investigated. However, the processes of physisorption involve difficulties to be commercially used due to 1) hydrogen storage that cannot achieve the standard (6 wt %) of minimum hydrogen storage suggested by Department of Energy (DOE) in the United States for practical use of hydrogen storage material, 2) low reproducibility of hydrogen storage, 3) harsh conditions required for hydrogen adsorption (extremely low temperature, for example, 100K or lower), and/or 4) relatively harsh conditions for hydrogen adsorption/desorption, and disintegration of materials being occurred during the course of adsorption/desorption of hydrogen.
Substances for hydrogen storage by chemisorptions may be classified into 1) alanate, LaNi type metal hydrides (50-100 KJ/mol), and 2) chemical hydrides (100 KJ/mol or more) such as NaBH4 and Ca(BH4)2. They store hydrogen by themselves in the internal structure of the materials, having strong adsorption energy of about 50 KJ/mol or more. However, they also involve difficulties to be commercially used due to 1) hydrogen storage that cannot achieve the standard (6 wt %) of minimum hydrogen storage suggested by Department of Energy (DOE) in the United States for practical use of hydrogen storage material, 2) low reproducibility after repeated adsorption/desorption of hydrogen and viability of disintegration of the structure, 3) difficulties in reversible reproduction, and/or 4) harsh conditions required for hydrogen adsorption/desorption (high temperature and/or high pressure).
In the technical field of hydrogen storage materials, those adsorbing hydrogen molecules by Kubas adsorption mechanism are considered as excellent storage media since they can adsorb hydrogen molecules at the temperature and pressure range close to ambient temperature and pressure, differently from conventionally reported materials adsorbing hydrogen on the basis of physisorption or chemisorption.
The monomeric or polymeric organometallic or coordination compounds and organic-transition metal hydride complexes, suggested by Hanwha Chemical R&D Center in Korean Patent Applications are suitable for commercial use because 1) they can store hydrogen with high capacity and high efficiency, 2) they can enable hydrogen adsorption/desorption under milder condition (for example, adsorption at 25° C., 30 atm and desorption at 100° C., 2 atm), and 3) there is no significant disintegration of the structure after repeated adsorption/desorption of hydrogen, as compared to hydrogen storage material conventionally suggested by means of Kubas binding. [See Korean Patent Application Nos. 10-2007-0090753, 10-2007-0090755, 10-2008-0020467, 10-2008-0078334.]
In case of said organic-transition metal hydride complexe, however, spontaneous aggregation of a part of the complex may occur owing to insufficient chemical stability, to be converted into a multimeric organic-transition metal complex having the molecular weight of an oligomer or more.